The present invention generally pertains to signal processing for a radio network of the type that includes at least one base station and at least one remote station and is particularly directed to processing signals received by a base station of the network to facilitate detection of received signals that resulted from propagation of signals transmitted by a remote station of the network.
This type of radio network is used for voice communications and for sending data. Typically the base stations of such a radio network are located at various, relatively stationary locations; and the remote stations are portable. A cellular radio network is an example of this type of radio network.
Each station of the network includes a radio device, such as a transceiver, and one or more signal processors for processing voice and/or data signals for transmission by the radio device and for processing signals received by the radio device in order to reconstruct voice and/or data signals from the received signals. Each station may be connected to one or more communication devices that are external to the radio network.
Communications within such radio networks are between remote stations and base stations, between different remote stations either directly or via one or more base stations, and between different base stations. The base stations control communications with each other and between the base stations and the remote stations.
Communications between base stations and remote stations are accomplished by the transmission of modulated signals over selected frequency channels. Because of the portability and limited size of the remote stations, signals transmitted over uplinks from remote stations sometimes are of relatively low power and thereby more distorted and difficult to detect than signals transmitted from base stations.
The uplink signals are distorted by frequency errors that result from propagation of the transmitted signals over the distance from the remote station to a base station. The frequency error is a function of the transmission path, relative motion between transmitter and receiver as well as any error in the frequency reference of the physical hardware. The uplink signals are also distorted by channel error effects, which are caused by background noise and time-varying multi-path attenuation and reflection of the transmitted signals.
Communications among a set of base stations within a particular radio network are conducted over a relatively high-power, more reliable communication link than the uplink from a remote station to the base station and thereby are not significantly affected by the distortions that are caused by propagation of the relatively low-power signals over the uplink from a remote station to a base station.
A signal processor is coupled to the base station for processing received signals in order to detect signals that resulted from propagation of signals transmitted by a remote station of the network. It is known to modulate signals for such transmission by embedding known information throughout the signals in a predetermined pattern; to encode signals for transmission from the remote stations of a particular radio network by processing the signals with a first feature, such as a pseudorandom number (PN); to process sample vectors of signals received by a base station of the network with a second feature that is related to the first feature, such as the conjugate of the PN used for encoding, in order to provide correlates of the sample vectors of the received signals; and to process the correlates with the known information in order to estimate whether the received signals resulted from propagation of a signal transmitted from a remote station in the particular network.
A signal is a particular sequence of modulated symbols that represent a message. A sample vector of a signal is a temporal sequence of measured values of the signal sampled over a predetermined duration.
It also is known to process the correlates of the signals to estimate frequency errors in the received signal and provide information sequences for the correlates that are corrected in accordance with the estimated frequency error. The corrected information sequences are processed to estimate any channel error effects in the received signal; and the corrected information sequences are compensated for the estimated channel error effects.